Enhanced hydrophilicity and aging resistance of zein via low-pressure oxygen Plasma: Role of localized plasticizing effects

This study investigates the effects of low-pressure oxygen cold plasma (LPOCP) treatments on the surface properties of zein films. LPOCP treatment achieved significant enhancements in hydrophilicity, reducing contact angles by up to 80 % and outperforming atmospheric-pressure plasma (APCP) treatments. Optimal results were obtained at 100 W for 60 s, yielding a contact angle of 9.5° ± 1.7° The vacuum conditions during LPOCP caused water loss from the zein matrix, and subsequent atmospheric exposure led to a non-uniform reabsorption of moisture. Surface hydrophilic groups, introduced by oxygen plasma, retained water near the surface, stabilizing functional groups and slowing hydrophobic recovery, as evidenced by sustained low contact angles over 14 days. Notably, LPOCP treatments preserved the surface morphology and secondary structure of the protein, unlike APCP treatments. Mechanical testing confirmed the role of a localized plasticizing effect due to altered water distribution, showing reduced stiffness (Young's modulus) while tensile strength and elongation at break remained unaffected. These findings highlight LPOCP as a promising tool for enhancing zein applicability in sustainable packaging and bioengineering applications.